Whipstocks have been in use for a long time to kick off deviations in a wellbore. A whipstock is a long insert in the wellbore which has a tapered component to guide a mill to cut through a casing an opening called a window. A lateral is then drilled through the window. Many times the production in an existing well is reduced to a level that warrants the drilling of a lateral into a producing formation to enhance production. In so doing, there is usually a production string in the wellbore. It, therefore, becomes desirable to be able to set a whipstock below the production string to kick off the lateral. In the future, subsequent laterals may be called for and retrievability can also become a significant feature. Without the ability to retrieve a whipstock even if inserted through tubing, any laterals to be initiated below the level of the permanent whipstock that has been set through tubing would require the milling out of the whipstock after removing the production tubing, and the placement of another whipstock at a lower location in the proper orientation for kicking off yet another lateral from the main wellbore.
There have been many anchoring systems employed for whipstocks. Some of these are illustrated in U.S. Pat. Nos. 1,812,880; 2,105,722; 2,170,284; 2,401,893; 2,445,100; 2,699,920; 5,035,292; and 5,277,251. Techniques have been developed for supporting a whipstock through the use of a cementitious material where the whipstock is installed through tubing. The whipstock can be repositioned for other laterals if the cement is milled out and the underlying sand layer is circulated out. This technique is illustrated in U.S. Pat. No. 5,423,387. A retrieval technique for a whipstock using a hook-shaped retrieval tool insertable into a slot in the whipstock is illustrated in U.S. Pat. No. 5,341,873.
Yet other patents are of general interest in the area of whipstock setting and milling techniques. Those patents include U.S. Pat. Nos. 5,193,620; 5,109,924; 2,725,936; 2,882,015; 2,211,803; and 5,474,126.
More recently, a permanent thru-tubing whipstock has been disclosed in U.S. Pat. No. 5,494,111.
The object of this invention is to improve the design illustrated in the U.S. Pat. No. 5,494,111 to provide a whipstock that is insertable through tubing and retrievable. It is another objective of the design to allow for the use of running and retrieval tools of known design to accomplish the setting, release, and retrieval.